Only in Japan!
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The Club Key (Japanese: クローバーキー, or クラブキー) is a green key in Densetsu no Starfy and Densetsu no Starfy 2. It is one of four special keys. The other three are the Heart Key, the Spade Key and the Diamond Key. They are all based on the four French playing card suits.
Densetsu no Starfy
In Densetsu no Starfy, the Club Key is found in the Sunken Ship. It was originally under the possession of Boctopus. It is one of four keys required to open Sunken Ship's prison cell and rescue Dolchina and Dolshi.
It is found through the top-right door of the main room (with the prison cell). When Starfy reaches the deck of the ship he finds the Club Key, only for a Kagiban to swallow the key and try to flee. Starfy must hit it three times with a Star Spin to defeat it and leave behind the key.
Densetsu no Starfy 2
In Densetsu no Starfy 2, the Club Key is found in Ogura Castle (Stage 11-4). It is one of four keys required to unlock the blue cage Moe was trapped in after he fell for an illusion of Ruby created by Ogura. It is found through the top-right door. Starfy must navigate a room with the Balloon. Like the room that contains the Diamond Key, Starfy must pass a room with several Stone Pillars. In this room is one of the two notes.